Subframe structure

ABSTRACT

A vehicular subframe structure comprises a light alloy member including a left side section and a right side section each extending in a front-rear direction of the vehicle and a cross section extending in a width direction of the vehicle, and a steel plate member including a left thinned plate section and a right thinned plate section each extending in the front-rear direction of the vehicle. When the light alloy member is stacked on the steel plate member they are joined together by friction stir welding from a side of the light alloy member, thereby forming a closed section.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of U.S. Ser. No.13/991,500, file 4 Jun. 2013, which is the US National Phase Applicationof International Application PCT/JP2011/078214, filed on 6 Dec. 2011,which claims priority to Japanese patent applications Nos. 2010-271337,2010-271339, and 2010-271340, all filed on 6 Dec. 2010, and 2011-010831,filed on 21 Jan. 2011. The entire subject matter of these prioritydocuments, including specification claims and drawings thereof, isincorporated by reference herein.

TECHNICAL FIELD

The present invention relates to a subframe structure mounted in thefront of a vehicle such as an automobile.

BACKGROUND ART

Vehicles such as automobiles use a subframe structure fixed to a frontside frame serving as a vehicle-body member, installed with a suspensioncomponent such as, e.g., a suspension arm and a stabilizer, and used forsupporting the suspension component.

As a subframe structure of this type, Patent Literature 1, e.g.,discloses one including a rear member made of light metal and installedwith a suspension component; two side members made of steel, joined tothe front ends of the two lateral sections of the rear member, andextending toward the front of a vehicle; and a cross member connectingthe two side members to each other in the width direction of thevehicle.

In addition, Patent Literature 2 discloses a vehicle subframe in which agrid-shaped die-cast subframe is configured to be divided into two andthe freedom degree of the shape of the dividing and joining portion canbe enhanced.

Further, Patent Literature 3 relates to the joining mechanism of anautomobile structure in connection with a center pillar and disclosesthe friction stir welding of a flange at an end edge on the side of theopening of a box-shaped structural member made of an aluminum alloy anda flat-plate-shaped cover made of a zinc steel plate.

Furthermore, Patent Literature 4 discloses a method for joiningdifferent types of metals together in which both materials made of thedifferent types of metals are superposed one on the other via a sealant,the deformation resistance of the sealant is reduced by heating todischarge the sealant interposed at the joining section from the joininginterface, and both the materials are joined together by resistancewelding or laser beam irradiation in a state in which both the materialsare brought into direct contact with each other. Furthermore, PatentLiterature 5 describes a method for joining different types of metalstogether by friction welding.

PRIOR ART REFERENCE Patent Literature

Patent Literature 1: JP 2007-302147 A

Patent Literature 2: JP 2006-347464 A

Patent Literature 3: JP 2009-126472 A

Patent Literature 4: JP 2008-23583 A

Patent Literature 5: JP 4134837 B2

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

Meanwhile, for its installation of a suspension component, a subframestructure disposed in the front of a vehicle is required to ensuredesired rigidity at the installation section. In addition, since thesubframe structure is disposed in the front of the vehicle, it isrequired to absorb an impact at the collision of the vehicle to preventthe impact from being transmitted into a passenger room. Moreover, it isrequired to achieve the weight reduction of the entire vehicle from theviewpoint of energy saving or the like.

Further, for example, it is assumed that the members made of thedifferent types of metals disclosed in Patent Literature 1 areintegrally joined together by the application of the joining methoddisclosed in Patent Literature 3. That is, it is assumed that flangesare provided at the end surfaces of the lateral sections of the rearmember made of light metal and at the end surfaces of the side membermade of steel, respectively, and the flange on the side of the rearmember and the flange on the side of the side member are joined togetherby the friction stir welding to construct the subframe structure.However, the subframe structure obtained by applying the joining methodof Patent Literature 3 to the structure of Patent Literature 1 givesrise to the problem that a closed cross section at the joining portioncannot be increased and desired rigidity and strength for supporting thesuspension component are hardly ensured.

Further, joining of the members made of the different types of metalstogether by the application of the joining method of Patent Literature 3to the structure of Patent Literature 1 gives rise to the problem thatthe temperature of a portion subjected to the friction stir welding isincreased and an electrodeposition coating film coated byelectrodeposition coating on the rear surface of the portion subjectedto the friction stir welding (the surface on a side opposite to thejoining surface between the different types of metals) is separated.

Furthermore, in a case in which the front subframe and the rear subframeof the subframe structure are joined together by melt welding withouthaving coatings applied thereto but they have the coatings appliedthereto afterwards, their structure becomes complicated, resulting in adifficulty in electrodeposition coating at their joining interfaces.

A general object of the present invention is to provide a subframestructure capable of ensuring desired rigidity and strength, enhancingshock absorption performance, and achieving a weight reduction.

A main object of the present invention is to provide the subframestructure capable of increasing closed cross sections at joiningportions and ensuring desired rigidity and strength.

Another object of the present invention is to provide the subframestructure capable of preventing the separation of electrodepositioncoating films at the rear surfaces of the joining portions even if thedifferent types of metals are joined together by friction stir welding.

Another object of the present invention is to provide the subframestructure capable of applying coatings to joining interfaces.

Means for Solving the Problem

In order to achieve the above objects, the present invention ischaracterized in that in a subframe structure for a vehicle, thesubframe structure being arranged at a front of the vehicle and fixed toor floatably supported by a vehicle-body member, including: a frontsubframe made of steel; and a rear subframe made of light metal, whereinthe rear subframe and the front subframe are divided in a front-reardirection of the vehicle, and the front subframe and the rear subframeare joined together by friction stir welding in a state in which therear subframe is superposed on the front subframe.

According to the present invention, the front subframe is made of steel,the rear subframe is made of light metal, and both the front subframeand the rear subframe are joined together by the friction stir welding.Thus, desired rigidity and strength for the installation or the like ofa suspension component such as a suspension arm can be ensured, andshock absorption performance at collision can be enhanced.

In addition, according to the present invention, the rear subframeincludes an aluminum die-cast body made of an aluminum alloy or thelike. Therefore, the weight reduction of the entire subframe structurecan be achieved. Moreover, according to the present invention, the rearmember conventionally including the two members of an upper member and alower member is integrated, and various reinforcing componentsconventionally provided inside the hollow rear member are integrallyformed by die-casting. Thus, with a reduction in the number ofcomponents, the weight reduction can be further achieved.

Further, according to the present invention, the left and right rearside sections of the rear subframe made of light metal such as, e.g., analuminum alloy are superposed on the upper surfaces of extendingsections formed in the front subframe made of steel to join flangesections together. Thus, the desired rigidity and strength for theinstallation of a suspension component such as a suspension arm can beensured, and shock absorption performance at collision can be enhanced.

Furthermore, according to the present invention, the rear subframeincluding the pair of left and right rear side sections and a rear crosssection is made of a light metal material such as, e.g., an aluminumalloy. Thus, the weight reduction can be further achieved than before.

Furthermore, according to the present invention, bolts penetratingclosed cross sections are fastened at non-joining portions at which thefront subframe and the rear subframe are not joined together, and thenon-joining sections at which welding is not allowed can be reinforcedby the fastening of the bolts. The front subframe and the rear subframeare firmly fixed together by the joining of the respective flangesections at their superposed portions, while the front subframe and therear subframe are fastened together by the bolts at the non-weldingportions not joined together. Thus, the rigidity and strength of theentire subframe structure can be further increased. As a result, even ina case in which the different types of the metals of the front subframemade of steel and the rear subframe made of light metal are mutuallyjoined together, the closed cross sections at the joining portions canbe increased and the desired rigidity and strength can be ensured incooperation with the bolt fastening portions serving as the non-joiningportions.

Furthermore, according to the present invention, the transfer offriction heat generated by the friction stir welding toelectrodeposition coating films formed on the lower surfaces of thinplates on the lower layer side is avoided, and the temperature of therear surfaces of the portions of the electrodeposition coating filmsformed on the lower surfaces of the thin plates on the lower layer sideis reduced. Thus, the separation of the electrodeposition coating filmsformed on the rear surfaces of the friction stir welding portions isprevented so that the electrodeposition coating films can be protected.

Furthermore, according to the present invention, the left and right sidemembers of the front subframe have the two or more steel thin platesjoined together, and the closed cross sections are formed between thejoined steel thin plates. Thus, the rigidity and strength can be furtherincreased.

Effect of the Invention

The present invention can provide a subframe structure capable ofensuring desired rigidity and strength, enhancing shock absorptionperformance, and achieving a weight reduction.

In addition, the present invention can provide the subframe structurecapable of increasing closed cross sections at joining portions andensuring desired rigidity and strength.

Further, the present invention can provide the subframe structurecapable of preventing the separation of electrodeposition coating filmsat the rear surfaces of the joining portions even if the different typesof metals are joined together by friction stir welding.

Furthermore, the present invention can provide the subframe structurecapable of applying coatings to joining interfaces.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic perspective view showing a state in which asubframe structure according to a first embodiment of the presentinvention is mounted in the front of an automobile;

FIG. 2 is an exploded perspective view of the subframe structureaccording to the first embodiment;

FIG. 3A is a plan view of the subframe structure according to the firstembodiment;

FIG. 3B is a partial plan view of a front subframe in a state in which arear subframe is removed from the subframe structure;

FIG. 4 is a vertical cross-sectional view taken along the line A-A inFIG. 3A;

FIG. 5 is a vertical cross-sectional view taken along the line B-B inFIG. 3A;

FIG. 6A is a perspective view showing a state in which friction stirwelding is performed using a joining tool;

FIG. 6B is a vertical cross-sectional view showing the state of thefriction stir welding;

FIGS. 7A to 7C are explanatory views showing a state in which sealantsremain in concave sections;

FIG. 8 is a schematic perspective view showing a state in which asubframe structure according to a second embodiment of the presentinvention is mounted in the front of the automobile;

FIG. 9 is an exploded perspective view of the subframe structureaccording to the second embodiment;

FIG. 10A is a plan view of the subframe structure according to thesecond embodiment;

FIG. 10B is a partial plan view of the front subframe in a state inwhich the rear subframe is removed from the subframe structure;

FIG. 11 is a vertical cross-sectional view taken along the line C-C inFIG. 10A;

FIG. 12 is a vertical cross-sectional view taken along the line D-D inFIG. 10A;

FIG. 13 is a schematic perspective view showing a state in which asubframe structure according to a third embodiment of the presentinvention is mounted in the front of the automobile;

FIG. 14 is an exploded perspective view of the subframe structureaccording to a third embodiment

FIG. 15A is a plan view of the subframe structure according to the thirdembodiment;

FIG. 15B is a partial plan view of the front subframe in a state inwhich the rear subframe is removed from the subframe structure;

FIG. 16 is a vertical cross-sectional view taken along the line E-E inFIG. 15A;

FIG. 17 is a vertical cross-sectional view taken along the line F-F inFIG. 15A;

FIG. 18A is a vertical cross-sectional view showing a state in which therespective flange sections of the front subframe and the rear subframeare joined together by the friction stir welding in the subframestructure according to the third embodiment;

FIG. 18B is a characteristic diagram in which the temperature of therear surfaces of friction stir welding portions is measured;

FIG. 18C is a vertical cross-sectional view showing a state after thefriction stir welding;

FIG. 19 is a plan view of a subframe structure according to a fourthembodiment;

FIG. 20 is a diagram showing the flow of the process of joining togetherthe front subframe and the rear subframe configuring the subframestructure by the friction stir welding in the first embodiment;

FIGS. 21A to 21C are views showing the process of joining the frontsubframe and the rear subframe together by the friction stir welding inthe first embodiment, in which FIG. 21A is a view showing the process ofsetting a workpiece; FIG. 21B is a view showing the process of applyingthe sealant; and FIG. 21C is a view showing the process of superposingthe workpieces one on the other;

FIGS. 22A to 22C are cross-sectional views schematically showing thedetails of a joining interface when the front subframe and the rearsubframe are joined together by the friction stir welding;

FIG. 23 is a perspective view showing a state in which the friction stirwelding is performed using the joining tool;

FIG. 24 is a horizontal cross-sectional view showing the joining sectionbetween the flange section of the front subframe and the flange sectionof the rear subframe;

FIG. 25A is a cross-sectional view showing a specific example of a statein which the respective flange sections of the front subframe and therear subframe are joined together by the friction stir welding in thesubframe structure according to the third embodiment;

FIG. 25B is a characteristic diagram in which the temperature of therear surfaces of friction stir welding portions is measured;

FIG. 25C is a cross-sectional view showing a state after the frictionstir welding;

FIG. 26 is a plan view of a subframe structure according to a fifthembodiment; and

FIGS. 27A to 27C are views showing the process of the friction stirwelding applied to the subframe structure according to the fifthembodiment, in which FIG. 27A is a cross-sectional view showing thestate of the start section of a location at which the friction stirwelding is started; FIG. 27B is a cross-sectional view showing a statebefore the friction stir welding at the end section of a location atwhich the friction stir welding is ended; and FIG. 27C is across-sectional view showing a state after the friction stir welding atthe end section of a location at which the friction stir welding isended.

EMBODIMENTS FOR CARRYING OUT THE INVENTION

Next, referring to the drawings as required, embodiments of the presentinvention will be described in detail. FIG. 1 is a schematic perspectiveview showing a state in which a subframe structure according to a firstembodiment of the present invention is mounted in the front of anautomobile. FIG. 2 is an exploded perspective view of the subframestructure according to the first embodiment. FIG. 3A is a plan view ofthe subframe structure according to the first embodiment. FIG. 3B is apartial plan view of a front subframe in a state in which a rearsubframe is removed from the subframe structure. FIG. 4 is a verticalcross-sectional view taken along the line A-A in FIG. 3A. FIG. 5 is avertical cross-sectional view taken along the line B-B in FIG. 3A.

As shown in FIG. 1, a subframe structure 10 according to the firstembodiment of the present invention is arranged at the front of avehicle body and provided so as to be fixed to a vehicle-body member(frame member) not shown or provided so as to be floatably supported bya floating mechanism not shown. Supporting the subframe structure 10 bythe floating mechanism not shown brings about the advantage thatvibration transmitted from the vehicle-body member can be suitablyabsorbed.

As shown in FIG. 1 to FIGS. 3A and 3B, the subframe structure 10 isdivided in the front-rear direction of the vehicle and includes a frontsubframe 12 made of steel and a rear subframe 14 made of light metal.Selected structural details of flange structures of the front and rearsubframes 12, 14 are omitted from FIGS. 1 and 2 of the drawings forillustrative purposes, but such structural details are shown in FIGS.4-7C. The front subframe 12 includes a press-formed body formed by,e.g., pressing of a steel plate member not shown. The rear subframe 14includes a die-cast body formed by, e.g., die casting in which analuminum alloy (aluminum) melted in the cavity of a die (die castingmachine) not shown is solidified.

Note that in each of the figures, “front” and “rear” represent the frontand rear sides of a vehicle 11 (see FIG. 1), respectively, in thefront-rear direction of the vehicle, and “left” and “right” representthe left and right sides of the vehicle 11, respectively, in the widthdirection of the vehicle.

As shown in FIG. 2, the front subframe 12 supports the vehiclefront-side of an engine 18 (see FIG. 1) via a front engine mount notshown and attached to a mount section (seat) 16, and has a front crossmember 20 that extends in the width direction of the vehicle and a pairof left and right side members 22 a and 22 b connected to both ends ofthe front cross member 20 along the axis direction thereof and extendingfrom the front cross member 20 to the rear of the vehicle in parallelwith each other.

Note that the front cross member 20 and the pair of left and right sidemembers 22 a and 22 b may be integrally formed by, e.g., casting,forging, or the like, or the front ends of the pair of left and rightside members 22 a and 22 b may be joined by welding to both ends of thefront cross member 20 along the axis direction thereof.

The front cross member 20 includes a hollow member made of a steelmaterial. In addition, front sections 24 a ahead of central sections(intermediate sections) 24 b of the pair of left and right side members22 a and 22 b along the axis direction thereof include hollow membersmade of a steel material. Moreover, the central sections 24 b of thepair of left and right side members 22 a and 22 b along the axisdirection thereof and rear sections 24 c behind the central sections 24b include thin plate sections 26 made thinner than the front sections 24a.

In this case, the thin plate sections 26 of the pair of left and rightside members 22 a and 22 b are formed as extending sections extending(elongating) by a prescribed length toward the rear side compared withconventional left and right side members. Further, as shown in FIG. 4,the central sections 24 b and the thin plate sections 26 of the pair ofleft and right side members 22 a and 22 b are formed by single thinplates to have substantially hat-like vertical cross sections, andflange sections 28 extending along the axis direction thereof are formedon both left and right sides of the left and right side members 22 a and22 b (although the right side member 22 b is omitted in FIG. 4).

The central sections 24 b of the pair of left and right side members 22a and 22 b along the axis direction thereof have bolt insertion holes 32formed therein for the insertion of bolts. In this case, as shown inFIG. 4, a pair of bolts 30 penetrates from the bottom side along thebolt insertion holes 32 of the left and right side members 22 a and 22 bso that screw sections 30 a of the bolts 30 can be fastened to bottomedscrew holes 34 provided at the front ends of the rear subframe 14. As aresult, the front subframe 12 and the rear subframe 14 are fixedtogether by the pair of bolts 30 at the positions on both left and rightsides along the width direction of the vehicle.

The rear subframe 14 includes a rear member supporting the vehiclerear-side of the engine 18 via a rear engine mount not shown andextending along the width direction of the vehicle. The rear portion ofthe front subframe 12 is coated on the respective upper surfaces of thecentral sections 24 b and the thin plate sections 26 behind the centralsections 24 b of the left and right side members 22 a and 22 b. The rearsubframe 14 includes a pair of left and right rear side sections 36 aand 36 b covering (superposing) some of the upper surfaces of the leftand right rear side members 22 a and 22 b, and a rear cross section 38connecting the pair of left and right rear side sections 36 a and 36 bto each other, with areas where the rear cross section 38 connects toeach of the side sections defining connecting portions 39, as shown inFIG. 3A. The rear subframe 14 is made of a light metal material such as,e.g., aluminum, magnesium, and the alloy of these substances.

The left and right rear side sections 36 a and 36 b have flange sections40 provided on both sides thereof, and the flange sections 40 are formedso as to extend from one end to the other end of the left and right rearside sections 36 a and 36 b along the axis direction thereof. In thiscase, lateral edge sections 40 a of the flange sections 40 of the leftand right rear side sections 36 a and 36 b are formed so as to slightlyprotrude toward both left and right sides along the width direction ofthe vehicle compared with the flange sections 28 of the left and rightside members 22 a and 22 b (see FIG. 5). The protruding lateral edgesections 40 a of the flange sections 40 of the left and right rear sidesections 36 a and 36 b have concave sections 42 recessed toward the topside and having ceiling surfaces 42 a (see FIGS. 7A-7C), and the concavesections 42 extend along the axis direction of the left and right rearside sections 36 a and 36 b.

In other words, the concave sections 42 having the ceiling surfaces 42 adefine grooves which are formed between the lateral edge sections 40 aand lateral end surfaces 28 a of the flange sections 28 of the left andright side members 22 a and 22 b. In addition, the flange sections 40are formed in such a manner that the lateral edge sections 40 a of theflange sections 40 of the left and right rear side sections 36 a and 36b (rear subframe 14) slightly protrude outwardly beyond adjacentportions of the left and right side members 22 a and 22 b along thewidth direction of the vehicle, respectively, compared with the flangesections 28 of the left and right side members 22 a and 22 b (frontsubframe 12) and then the protruding lateral edge sections extend towardthe vertically downward direction beyond the upper surfaces of theflange sections of the left and right side members 22 a and 22 b, asshown.

Note, with reference to FIG. 7C that the lower surfaces of the lateraledge sections 40 a, which extend toward the vertically downwarddirection, of the flange sections 40 of the left and right rear sidesections 36 a and 36 b may be provided so as to be on the same orsubstantially the same level as the lower surfaces of the flangesections 28 of the left and right side members 22 a and 22 b along thehorizontal direction when the welding operation between the flanges 40,28 is concluded.

In this case, the flange sections 28 provided on both left and rightsides of the left and right side members 22 a and 22 b are positioned onthe lower side, and the flange sections 40 provided on both left andright sides of the left and right rear side sections 36 a and 36 b arepositioned on the upper side. When the flange sections 28 and 40 areintegrally joined together by friction stir welding in their superposedstate, closed cross sections 44 are formed (see FIG. 4 and FIG. 5).

In addition, the left and right side members 22 a and 22 b and the leftand right rear side sections 36 a and 36 b of the rear member arefastened together when the bolts 30 inserted into the bolt insertionholes 32 provided at the central sections are screwed into the screwholes 34 provided in the left and right rear side sections 36 a and 36 bso as to penetrate the closed cross sections 44.

Inside the closed cross sections 44, collar members 46 are provided thatinclude cylindrical bodies surrounding the peripheral surfaces of thebolts 30 and reinforce the joining strength between the left and rightside members 22 a and 22 b and the left and right rear side sections 36a and 36 b when the bolts 30 are fastened. The bolts are fastened atnon-joining portions at which the front subframe 12 and the rearsubframe 14 are not joined together by the friction stir welding thatwill be described later, and the non-joining portions at which weldingis not allowed can be reinforced by the fastening of the bolts. As aresult, even in a case in which the front subframe 12 made of steel andthe rear subframe 14 made of light metal are mutually joined together bythe friction stir welding, desired rigidity and strength can be ensuredin cooperation with bolt fastening portions serving as the non-joiningportions.

Accordingly, the front subframe 12 and the rear subframe 14 are firmlyfixed (joined) together when the respective flange sections 28 and 40are joined together by the friction stir welding at their superposedportions. In addition, the front subframe 12 and the rear subframe 14are fastened together by the bolts 30 at the non-welding portions notsubjected to the friction stir welding. Thus, the rigidity and strengthof the entire subframe structure 10 can be further increased. Note thatif female screw holes (not shown) are formed at positions behind thefastening portions of the bolts 30 in the left and right rear sidesections 36 a and 36 b and reinforcing bolts, not shown, are insertedfrom below the rear sections 24 c of the left and right side members 22a and 22 b so as to be fastened to the female screw holes, the rigidityand strength can be further increased.

The subframe structure 10 according to the first embodiment is basicallyconfigured as described above. Next, the functions and effects of thesubframe structure will be described. FIG. 6A is a perspective viewshowing a state in which the friction stir welding is performed using ajoining tool. FIG. 6B is a vertical cross-sectional view showing thestate of the friction stir welding.

First, a description will be given of the process of integrally joiningtogether the superposed portions between the flange sections 28 on theside of the front subframe 12 made of a steel material and the flangesections 40 on the side of the rear subframe 14 made of an aluminumalloy material by the friction stir welding.

As shown in FIGS. 6A and 6B, a joining tool 50 for use in the frictionstir welding has a cylindrical rotor (Stir Rod) 52 rotated and drivenabout a rotating shaft by a rotating and driving source such as a motornot shown and has a joining pin (Probe) 54 protruding from the bottomcenter of the rotor 52 along the direction of the shaft. The diameter ofthe joining pin 54 is set to be smaller than that of the rotor 52, and ashoulder section 56 is formed at the annular step section between thejoining pin 54 and the rotor 52.

Next, the process of joining the front subframe 12 and the rear subframe14 together will be described. Note that the front subframe 12 includesa press-formed body formed by pressing a steel plate member, while therear subframe 14 includes a die-cast body formed by die-casting with analuminum alloy.

First, the front subframe 12 is mounted on a clamp board not shown, andthen sealants 58 (e.g., air-dry sealants) are applied onto the uppersurfaces of the front subframe 12 by a sealant application mechanism notshown. After the rear subframe 14 is coated on the upper surfaces (thethin plate sections 26 behind the central sections 24 b) of the frontsubframe 12 having the sealants 58 applied thereto, the front subframe12 and the rear subframe 14 superposed in the top-bottom direction areclamped by a clamp mechanism not shown.

Subsequently, the flange sections 28 of the front subframe 12 and theflange sections 40 of the rear subframe 14 are joined together by thefriction stir welding using the joining tool 50 described above. Notethat jigs 60 for supporting welding force to be applied to therespective flange sections 28 and 40 by the joining tool 50 are providedbeneath the respective flange sections 28 and 40 of the front subframe12 and the rear subframe 14.

Next, the outline of the process of the friction stir welding is asfollows. Note that the details of the process of the friction stirwelding will be described later.

The rotor 52 and the joining pin 54 is caused to gradually come close tothe upper surfaces of the rear subframe 14 made of a light metalmaterial such as an aluminum alloy while being integrally rotated by therotating and driving source not shown, and then the tip end of thejoining pin 54 is brought into contact with the upper surfaces of therear subframe 14 by welding force (pressing force) so as be rotated topenetrate. Thus, plastic flow areas are generated in the rear subframe14.

Moreover, the rotor 52 and the joining pin 54 are pressed to penetratewhile being integrally rotated, and the joining pin 54 is inserted inthe vertically downward direction until the shoulder section 56 of therotor 52 slides on the upper surfaces of the rear subframe 14. On thisoccasion, the welding force is applied until the tip end of the joiningpin 54 is brought into contact with the upper surfaces of the frontsubframe 12 made of a steel material.

When the joining pin 54 is rotated to penetrate until it is brought intocontact with the upper surfaces of the front subframe 12, the plasticflow areas generated in the rear subframe 14 made of a light metalmaterial are plastically flowed and the new surfaces of the steel platesof the front subframe 12 made of a steel material are exposed. Thus, thefront subframe 12 is solid-phase welded to the rear subframe 14.

As described above, when the rotor 52 and the joining pin 54 aredisplaced along the axis direction of the superposed flange sections 28and 40 while maintaining a state in which the rotor 52 and the joiningpin 54 are rotated to penetrate and the tip end of the joining pin 54 isbrought into contact with the upper surfaces of the front subframe 12,friction stir welding portions 62 (see netted sections in FIG. 3A) areformed. Note that at the friction stir welding portions 62,intermetallic compounds are generated at the joining interfaces betweenthe rear subframe 14 (light metal material such as an aluminum alloy) onthe top side and the front subframe 12 (steel material) on the bottomside. The intermetallic compounds are generated so as to be dispersed inthe joining interfaces in a granular form or a divided layered formrather than a continuous layered form extending over the entire joininginterfaces.

In the first embodiment, the front subframe 12 includes a press-formedbody made of steel, and the rear subframe 14 includes a die-cast bodymade of light metal. Thus, desired rigidity and strength can be ensuredat the installation or the like of a suspension component such as asuspension arm not shown, and shock absorption performance at collisioncan be enhanced.

In addition, in the first embodiment, the rear subframe 14 includes analuminum die-cast body made of, e.g., an aluminum alloy or the like.Thus, the weight reduction of the entire subframe structure 10 can beachieved. Moreover, in the first embodiment, the rear memberconventionally including two members, i.e., upper and lower members isintegrated, and various reinforcing components provided in the hollowrear member are integrally formed by die-casting. Thus, with a reductionin the number of components, the weight reduction can be furtherachieved.

Further, in the first embodiment, the left and right rear side sections36 a and 36 b of the rear subframe 14 made of light metal such as, e.g.,an aluminum alloy are superposed on the upper surfaces of the thin platesections (extending sections) 26 having the substantially hat-likevertical cross sections formed in the front subframe 12 made of steel tojoin the flange sections 28 and 40 together. Thus, desired rigidity andstrength can be ensured at the installation or the like of a suspensioncomponent such as the suspension arm not shown, and shock absorptionperformance at collision can be enhanced.

Furthermore, in the first embodiment, the rear subframe 14 having thepair of left and right rear side sections 36 a and 36 b and the rearcross section 38 is made of a light metal material such as, e.g., analuminum alloy. Thus, the weight reduction can be further achieved thanbefore.

FIGS. 7A to 7C are explanatory views showing a state in which thesealants remain in the concave sections.

Hereinafter, a description will be given, based on FIGS. 7A-7C, of asealant remaining structure in which the sealants 58 interposed betweenthe front subframe 12 and the rear subframe 14 are protruded from bothleft and right sides and remain in the concave sections 42.

When the rear subframe 14 is superposed on the front subframe 12 havingthe sealants 58 applied onto the upper surfaces thereof (see FIG. 7A)and then the front and rear subframes are clamped by the clamp mechanismnot shown, the sealants 58 are slightly protruded from both left andright sides of the front subframe 12 and the rear subframe 14 (see FIG.7B).

The sealants 58 protruded from both left and right sides of thesuperposed front subframe 12 and the rear subframe 14 remain in theconcave sections 42 having the ceiling surfaces 42 a. Moreover, when thefront subframe 12 and the rear subframe 14 are joined together by thefriction stir welding in their clamped state, the sealants 58 arefurther protruded from both left and right sides. As a result, thenecessary and sufficient amount of the sealants 58 is held in theconcave sections 42 (see FIG. 7C).

If the sealants 58 held in the concave sections 42 include, e.g.,air-dry sealants, they solidify after the lapse of a prescribed periodof time to seal the gaps on the left and right sides of the frontsubframe 12 and the rear subframe 14. As a result, in the embodiment,the scattering of the sealants 58 protruded from both left and rightsides of the front subframe 12 and the rear subframe 14 joined togetherby the friction stir welding is prevented, and the intrusion of waterfrom the gaps on both left and right sides of the front subframe 12 andthe rear subframe 14 is prevented. Thus, high antirust performance canbe ensured.

In addition, an operator can visually confirm the remaining degree(remaining amount) of the sealants 58 in the concave sections 42 fromthe outside. Therefore, by confirming the application amount of thesealants 58, the operator can determine whether the sealants 58 havebeen reliably interposed between the front subframe 12 and the rearsubframe 14.

Moreover, although the closed cross sections 44 are formed between thefront subframe 12 and the rear subframe 14 when the flange sections 28and 40 are joined together by the friction stir welding, the sealants 58are also protruded toward areas inside the flange sections 28 and 40where the closed cross sections are formed and solidified to exhibit thesealing function (see FIG. 6B). Thus, water remaining preventionstructure can be obtained in which water droplets do not remain in thegaps between the respective flange sections 28 and 40 even if the waterdroplets (water) fall down along the inner wall surfaces of the rearsubframe 14.

Furthermore, when the different types of the materials of the frontsubframe 12 made of a steel member and the rear subframe 14 made of analuminum member are joined together by the friction stir welding, thereis a concern that a potential difference occurs between the respectivemetal materials due to a difference in the ionization of the respectivemetal materials and corrosion is caused by the contact between thedifferent types of the metal materials when corrosion current flows.However, in the embodiment, the flow of corrosion current can beprevented by the solidification of the sealants 58 protruded from theflange sections 28 and 40 joined together by the friction stir welding.As a result, in the embodiment, resistance to corrosion caused by thecontact between the different types of the metal materials can beenhanced.

Next, a subframe structure 100 according to a second embodiment of thepresent invention will be described below. Note that in the followingembodiment, the same constituents as those of the subframe structure 10according to the first embodiment shown in FIG. 1 will be denoted by thesame reference symbols and their detailed descriptions will be omitted.

FIG. 8 is a schematic perspective view showing a state in which thesubframe structure according to the second embodiment of the presentinvention is mounted in the front of the automobile. FIG. 9 is anexploded perspective view of the subframe structure according to thesecond embodiment. FIG. 10A is a plan view of the subframe structureaccording to the second embodiment. FIG. 10B is a partial plan view ofthe front subframe in a state in which the rear subframe is removed fromthe subframe structure. FIG. 11 is a vertical cross-sectional view takenalong the line C-C in FIG. 10A. FIG. 12 is a vertical cross-sectionalview taken along the line D-D in FIG. 10A.

In the subframe structure 100 according to the second embodiment, asshown in FIG. 11, bolt fastening portions at the central sections 24 bof the left and right side members 22 a and 22 b of the front subframe12 have the closed cross sections 44 formed when two thin plates 102 aand 102 b made of a steel material are joined together. Accordingly, thesubframe structure 100 according to the second embodiment is differentfrom the subframe structure 10 according to the first embodiment inwhich the bolt fastening portions of the left and right side members 22a and 22 b each include the single steel plate and the closed crosssections 44 (see FIG. 4) are formed between the front subframe 12 andthe rear subframe 14.

In this case, the two thin plates 102 a and 102 b configuring the leftand right side members 22 a and 22 b have the bolt insertion holes 32and 32 for the insertion of the bolts 30. The bolt insertion holes 32and 32 are provided so that the screw sections 30 a of the bolts 30inserted along the bolt insertion holes 32 and 32 penetrate the closedcross sections 44 formed by the two thin plates 102 and 102 b whenscrewed into the screw holes 34 of the rear subframe 14.

Note that in the closed cross sections 44, there are provided collarmembers 104 each including a cylindrical body surrounding the peripheralsurface of the bolt 30 and having one end thereof connected to the onethin plate 102 a along the axis direction thereof and the other endthereof connected to the other thin plate 102 b along the axis directionthereof. The collar members 104 are provided to prevent the deformationof the thin plates 102 a and 102 b due to the fastening of the bolts 30and reinforce joining strength at the bolt fastening portions. In thiscase, the collar members 104 may be integrally formed with the lowerthin plates 102 b or may be welded in advance to the upper surfaces ofthe thin plates 102 for fixation. In addition, for the fastening of thebolts 30 penetrating the closed cross sections 44 formed by the two thinplates 102 a and 102 b, peripheral bolt fastening portions may be weldedin which the rear subframe 14 made of an aluminum alloy material and theupper thin plates 102 made of a steel material are laminated (see FIG.11).

In the second embodiment, the two thin plates 102 a and 102 b made of asteel material are joined together to form closed cross sections 44 inthe left and right side members 22 a and 22 b, which brings about theadvantage that the closed cross-sectional areas can be increased. As aresult, the rigidity and strength can be further increased.

Next, a subframe structure 200 according to a third embodiment of thepresent invention will be described below.

FIG. 13 is a schematic perspective view showing a state in which thesubframe structure according to the third embodiment of the presentinvention is mounted in the front of the automobile. FIG. 14 is anexploded perspective view of the subframe structure according to thethird embodiment. FIG. 15A is a plan view of the subframe structureaccording to the third embodiment. FIG. 15B is a partial plan view ofthe front subframe in a state in which the rear subframe is removed fromthe subframe structure. FIG. 16 is a vertical cross-sectional view takenalong the line E-E in FIG. 15A. FIG. 17 is a vertical cross-sectionalview taken along the line F-F in FIG. 15A. FIG. 18A is a verticalcross-sectional view showing a state in which the respective flangesections of the front subframe and the rear subframe are joined togetherby the friction stir welding in the subframe structure according to thethird embodiment. FIG. 18B is a characteristic diagram in which thetemperature of the rear surfaces of friction stir welding portions ismeasured. FIG. 18C is a vertical cross-sectional view showing a stateafter the friction stir welding.

As shown in FIG. 14, the subframe structure 200 according to the thirdembodiment is different from the subframe structures 10 and 100according to the first and second embodiments in that sections rangingfrom the central sections 24 b of the left and right side members 22 aand 22 b configuring the front subframe 12 to extending sections 202(including flange sections 204 a and 204 b) behind the central sections24 b are made thin by the lamination of two thin plates 206 a and 206 bmade of a steel material and that the entire left and right side members22 a and 22 b including the extending sections 202 include the two thinplates 206 a and 206 b.

In this case, before the front subframe 12 and the rear subframe 14 arejoined together by the friction stir welding, electrodeposition coatingfilms 208 a to 208 c are formed by electrodeposition coating processingon both the front and rear surfaces and the joining surfaces (laminatingsurfaces) between both the front and rear surfaces of the flangesections 204 a and 204 b of the left and right side members 22 a and 22b (see FIG. 18A).

The flange sections 204 a and 204 b of the left and right side members22 a and 22 b having the two thin plates 206 a and 206 b laminatedthereon as described above and the left and right side sections 36 a and36 b of the rear subframe 14 are joined together by the friction stirwelding using the joining tool 50. On this occasion, the joining pin 54of the joining tool 50 is rotated to penetrate the left and right sidesections 36 a and 36 b and brought into contact with the flange sections204 a and 204 b of the left and right side members 22 a and 22 b, whichresults in the application of friction heat to the left and right sidesections 36 a and 36 b. However, since the left and right side members22 a and 22 b include the two laminated thin plates 206 a and 206 b madeof steel at the rear surfaces 210 of the friction stir welding portions,the temperature of the electrodeposition coating films 208 c does notreach prescribed temperature (threshold temperature) at which thedecomposition of the electrodeposition coating films 208 c is allowed(see FIG. 18B). As a result, the separation of the electrodepositioncoating films 208 c can be prevented (see FIG. 18C).

In other words, friction heat is generated when the joining pin 54 isrotated to penetrate toward the joining objects at the friction stirwelding, and the electrodeposition coating films 208 c formed on thelower surfaces of the thin plates 206 b on the lower layer side out ofthe two thin plates 206 a and 206 b made of steel may be separated. Inthe third embodiment, the sections ranging from the central sections 24b of the left and right side members 22 a and 22 b configuring the frontsubframe 12 to the extending sections 202 behind the central sections 24b are made thin by the lamination of the two thin plates 206 a and 206 bmade of a steel material, the transfer of friction heat to theelectrodeposition coating films 208 c formed on the lower surfaces ofthe thin plates 206 b on the lower layer side is avoided, and thetemperature of the portions of the electrodeposition coating filmsformed on the lower surfaces of the thin plates 206 on the lower layerside is reduced. Thus, the electrodeposition coating films 208 c formedon the rear surfaces 210 of the friction stir welding portions areprotected.

FIG. 18B is the characteristic diagram in which the temperature of therear surfaces 210 (the lower surfaces of the thin plates 206 b on thelower layer side out of the two laminated thin plates 206 a and 206 bmade of steel) of the friction stir welding portions is measured using atemperature sensor not shown. In this case, although the temperature ofthe lower surfaces of the thin plates 206 b on the lower layer sideslightly increases due to the friction stir welding, the temperature ofthe electrodeposition coating films 208 c does not reach the prescribedtemperature (threshold temperature) at which the electrodepositioncoating films 208 c formed on the lower surfaces of the thin plates 206b on the lower layer side are decomposed. Therefore, since theseparation of the electrodeposition coating films 208 c is prevented,the electrodeposition coating films 208 can be stably protected.

Note that at the joining surfaces between the front subframe 12 and therear subframe 14, the electrodeposition coating films 208 a formedbetween the thin plates 206 a on the upper layer side out of the twolaminated thin plates 206 a and 206 b made of steel and the rearsubframe 14 made of light metal such as an aluminum alloy can bereliably extruded outside the joining surfaces by the friction stirwelding.

In addition, the third embodiment exemplifies the structure in which thesections ranging from the central sections 24 b of the left and rightside members 22 a and 22 b configuring the front subframe 12 to theextending sections 202 (including the flange sections 204 a and 204 b)behind the central sections 24 b are formed by the lamination of the twothin plates 206 a and 206 b made of a steel material. However, the thirdembodiment is not limited to the structure, and the number of thinplates may be two or more.

FIG. 19 is a plan view of a subframe structure according to a fourthembodiment.

A subframe structure 300 according to the fourth embodiment ischaracterized in that front ends 302 of the left and right rear sidesections 36 a and 36 b made of an aluminum alloy material are inclinedso as to cross an axial line G of the rear cross section 38. Theinclination of the front ends 302 brings about the advantage that thelengths and cross-sectional areas of the friction stir welding portions62 can be arbitrarily increased and decreased for adjustment. Note thatas the shapes of the front ends 302, the inside of the respective rearside sections 36 a and 36 b may be longer than the outside thereoftoward the front direction or the outside may be longer than the insidetoward the front direction.

Next, a joining method in each of the embodiments will be described indetail below.

FIG. 20 is a diagram showing the flow of the process of joining togetherthe front subframe 12 and the rear subframe 14 configuring the subframestructure 10 by the friction stir welding in the first embodiment.

First, a description will be given, with reference to FIG. 20, of theprocess of integrally joining together the superposed portions betweenthe flange sections 28 on the side of the front subframe 12 made of asteel material and the flange sections 40 on the side of the rearsubframe 14 made of a light metal material such as an aluminum alloy bythe friction stir welding.

FIGS. 21A-21C are views showing the process of joining the frontsubframe 12 and the rear subframe 14 together by the friction stirwelding. FIG. 21A is a view showing the process of setting the workpiece(S1 in FIG. 20). FIG. 21B is a view showing the process of applying thesealant (S2 in FIG. 20). FIG. 21C is a view showing the process ofsuperposing the workpieces one on the other (S3 in FIG. 20). FIGS. 22Ato 22C are cross-sectional views schematically showing the details ofthe joining interfaces when the front subframe 12 and the rear subframe14 are joined together by the friction stir welding.

First, a press-formed body 12′ formed into the front subframe 12 using asteel material (see FIG. 22A) is subjected to zinc alloy plating 12 mand then to cation electrodeposition coating 12 d. As shown in FIG. 22A,the front subframe 12 of the workpiece having been subjected to the zincalloy plating 12 m and the cation electrodeposition coating 12 d is seton the jigs 60 such as clamp boards (S1 in FIG. 20).

Next, as shown in FIG. 21B, the sealants 58, e.g., the air-dry sealantsare applied onto the upper surfaces of the flange sections 28 of thefront subframe 12 by the sealant application mechanism not shown (seeFIG. 22A) (S2 in FIG. 20).

Then, as shown in FIG. 21C, the flange sections 40 of the die-cast rearsubframe 14 of the workpiece made of light metal such as an aluminumalloy material is superposed on the flange sections 28 having thesealants 58 applied onto the upper surfaces of the front subframe 12,and the flange sections 28 and 40 are clamped by the clamp mechanism notshown (S3 in FIG. 20). At this time, as shown in FIG. 22B, the sealants58 spread between the flange sections 28 of the front subframe 12 andthe flange sections 40 of the rear subframe 14.

Next, the process of joining the front subframe 12 and the rear subframe14 together (the process of performing the friction stir welding andextruding the sealants 58) in step S4 of FIG. 20 is performed asfollows.

The flange sections 28 of the front subframe 12 and the flange sections40 of the rear subframe 14 are joined together by the friction stirwelding using the joining tool 50. Note that as described above, thejigs 60 for receiving welding force to be applied to the respectiveflange sections 28 and 40 by the joining tool 50 are provided beneaththe respective flange sections 28 and 40 of the front subframe 12 andthe rear subframe 14.

As shown in FIG. 21C, the rotor 52 and the joining pin 54 are caused togradually come close to the upper surfaces of the flange sections 40 ofthe rear subframe 14 made of a light metal material such as an aluminumalloy while being integrally rotated by the rotating and driving sourcenot shown, and the tip end of the joining pin 54 is brought into contactwith the upper surfaces of the flange sections 40 of the rear subframe14 by welding force (pressing force) so as be rotated to penetrate.Thus, plastic flow areas are generated in the flange sections 40 of therear subframe 14 (see FIG. 22C). By the plastic flow, intermetalliccompounds kc as the compounds of light metal (e.g., aluminum) and ironare formed.

FIG. 23 is a perspective view showing a state in which the friction stirwelding is performed using the joining tool.

Moreover, the rotor 52 and the joining pin 54 are pressed to penetratethe flange sections 40 of the rear subframe 14 while being integrallyrotated, and the joining pin 54 is inserted toward the verticallydownward direction until the shoulder section 56 of the rotor 52 slideson the upper surfaces of the flange sections 40 of the rear subframe 14as shown in FIG. 23.

On this occasion, as shown in FIG. 22C, the welding force is applieduntil, after penetrating the flange sections 40 of the rear subframe 14,the tip end of the joining pin 54 breaks through the layers of theapplied sealants 58, the layers subjected to the cationelectrodeposition coating 12 d, and the layers subjected to the zincalloy plating 12 m formed on the upper surfaces of the flange sections28 of the front subframe 12; extrudes the layers of the sealants 58, thelayers subjected to the cation electrodeposition coating 12 d, and thelayers subjected to the zinc alloy plating 12 m to the peripheries ofthe joining surfaces between the flange sections 40 and 28; and isbrought into direct contact with the upper surfaces of the flangesections 28 of the front subframe 12.

When the joining pin 54 is rotated to penetrate until it is brought intocontact with the upper surfaces of the front subframe 12 as describedabove, the plastic flow areas sr generated in the flange sections 40 ofthe rear subframe 14 made of a light metal material are plasticallyflowed and the new surfaces of the steel plates of the front subframe 12made of a steel material are exposed to form the intermetallic compoundskc after the layers of the sealants 58, the layers subjected to thecation electrodeposition coating 12 d, and the layers subjected to thezinc alloy plating 12 m are extruded. Thus, the front subframe 12 issolid-phase welded to the rear subframe 14.

That is, since the rear subframe 14 made of a light metal material andthe flange sections 28 of the front subframe 12 are firmly fixedtogether in such a manner that the antioxidants of the zinc alloyplating 12 m, the coating films of the cation electrodeposition coating12 d, and the sealants 58 are extruded to the peripheries of the joiningsurfaces and mixed together to form walls, the separation of coatings orthe like is prevented. In addition, the sealants 58, the layerssubjected to the cation electrodeposition coating 12 d, and the layerssubjected to the zinc alloy plating 12 m do not exist at the joiningsurfaces between the flange sections 40 and the flange sections 28. Notethat as described above, mixtures m of the layers subjected to the zincalloy plating 12 m, the layers subjected to the cation electrodepositioncoating 12 d, and the sealants 58 are formed like walls around thejoining pin 54.

As described above, when the rotor 52 and the joining pin 54 are rotatedto penetrate the flange sections 40 of the rear subframe 14 anddisplaced along the extending direction of the superposed flangesections 28 and 40 in a state in which the tip end of the joining pin 54is brought into contact with the upper surfaces of the flange sections28 of the front subframe 12, the friction stir welding portions 62 (seethe netted sections in FIG. 3A) are formed.

Note that at the friction stir welding portions 62, the intermetalliccompounds kc are generated at the joining interfaces between the rearsubframe 14 (light metal material such as an aluminum alloy) on the topside and the front subframe 12 (steel material) on the bottom side asshown in FIG. 22C. The intermetallic compounds kc are generated so as tobe dispersed in the joining interfaces in a granular form or a dividedlayered form rather than a continuous layered form extending over theentire joining interfaces.

FIG. 24 is a horizontal cross-sectional view showing the joining sectionbetween the flange section 28 of the front subframe 12 and the flangesection 40 of the rear subframe 14.

If the sealants 58 held in the concave sections 42 include, e.g.,air-dry sealants, they solidify after the lapse of a prescribed periodof time to reliably seal the gaps between the flange sections 28 and 42on the left and right sides of the front subframe 12 and the rearsubframe 14.

As a result, in the embodiment, the scattering of the sealants 58protruded from both left and right sides of the front subframe 12 andthe rear subframe 14 joined together by the friction stir welding isprevented since the sealants 58 remain in the concave sections 42. Thus,the reliability of the filling of the sealants 58 can be achieved.

In addition, the intrusion of corrosion factors such as water from thegaps on both left and right sides of the front subframe 12 and the rearsubframe 14 is reduced. Thus, high antirust performance can be ensured.

Moreover, the operator can visually confirm the remaining degree(remaining amount) of the sealants 58 in the concave sections 42 fromthe outside. Therefore, by confirming the application amount of thesealants 58, the operator can determine whether the sealants 58 havebeen reliably interposed between the front subframe 12 and the rearsubframe 14.

Further, although closed space having the closed cross sections 44 isformed between the front subframe 12 and the rear subframe 14 when theflange sections 28 and 40 are joined together by the friction stirwelding, the sealants 58 are also protruded toward the areas inside theflange sections 28 and 40 where the closed space having the closed crosssections is formed and solidified to exhibit the sealing function. Thus,the water remaining prevention structure can be obtained in which waterdroplets flow on the protruded sealants 58 between the respective flangesections 28 and 40 and do not remain in the gaps between the flangesections 28 and 40 even if the water droplets (water) fall down alongthe inner wall surfaces of the rear subframe 14 on the top side asindicated by the arrow al in FIG. 24.

Furthermore, when the different types of the materials of the frontsubframe 12 made of a steel member and the rear subframe 14 made of alight metal member such as aluminum are joined together by the frictionstir welding, there is a concern that a potential difference occursbetween the respective metal materials due to a difference in theionization of the respective metal materials and corrosion is caused bythe contact between the different types of the metal materials whencorrosion current flows. However, in the embodiment, the flow ofcorrosion current can be prevented by the solidification of the sealants58 protruded from the flange sections 28 and 40 joined together by thefriction stir welding. As a result, resistance to corrosion caused bythe contact between the different types of the metal materials can beenhanced.

In addition, since the front subframe 12 can have a coating appliedthereon in its single state, the coating is facilitated and the labor ofthe coating is greatly saved. Further, the omission of the coating ofthe front subframe 12 is prevented.

Next, a joining method in the third embodiment will be described indetail below.

As shown in FIG. 25A, the flange sections 204 a and 204 b of the leftand right side members 22 a and 22 b having the two thin plates 206 aand 206 b laminated thereon and the flange sections 40 and 40 of theleft and right side sections 36 a and 36 b of the rear subframe 14 arejoined together by the friction stir welding using the joining tool 50.At this time, the joining pin 54 of the joining tool 50 is, when broughtinto contact with the flange sections 204 a and 204 b, rotated topenetrate the flange sections 40 and 40 of the left and right sidesections 36 a and 36 b to generate the plastic flow areas sr andextrudes the electrodeposition coating films 208 a and the sealants 258of the left and right side members 22 a and 22 b to the peripheries ofthe joining surfaces between the flange sections 204 a and 204 b and theflange sections 40 and 40 to form the walls of the mixtures m of theelectrodeposition coating films 208 a and the sealants 258.

On this occasion, the electrodeposition coating films 208 a and thesealants 258 are extruded to the peripheries of the joining surfaces toform the walls of the mixtures m, and the flange sections 40 and 40 ofthe left and right side sections 36 a and 36 b and the flange sections204 a and 204 b of the left and right side members 22 a and 22 b arefirmly fixed together when the intermetallic compounds kc as thecompounds of light metal (e.g., aluminum) and iron are formed by plasticflow. Therefore, the separation of the electrodeposition coating films208 is prevented. In addition, the electrodeposition coating films 208 aand the sealants 258 do not exist at the respective joining surfacesbetween the flange sections 40 and 40 and the thin plates 206 a and 206b.

Moreover, although friction heat is applied to the flange sections 40and 40 of the left and right side sections 36 a and 36 b, thetransmissibility of the heat to the rear surfaces 210 of the frictionstir welding portions (the rear surfaces of the flange sections 204 aand 204 b) is reduced by the lamination of the two thin plates 206 a and206 b made of steel. Therefore, the temperature of the electrodepositioncoating films 208 c formed on the rear surfaces of the flange sections204 a and 204 b does not reach prescribed temperature (thresholdtemperature) at which the decomposition of the electrodeposition films208 c is allowed (see FIG. 25B). As a result, the separation of theelectrodeposition films 208 c from the rear surfaces of the flangesections 204 a and 204 b can be prevented (see FIG. 25C).

In other words, friction heat is generated when the joining pin 54 isrotated to penetrate toward the joining objects at the friction stirwelding, and the electrodeposition coating films formed on the lowersurfaces of the laminated thin plates made of steel may be separated.

In view of this, in the third embodiment, the sections ranging from thecentral sections 24 b to the extending sections 202 behind the centralsections 24 b of the left and right side members 22 a and 22 bconfiguring the front subframe 12 are formed to have air space thereinin such a manner that the two thin plates 206 a and 206 b made of asteel material are made thin and laminated together as shown in FIGS.25A-25 to reduce the transmissibility of the heat.

Thus, the transfer of the friction heat generated when the joining pin54 is rotated to penetrate to the electrodeposition coating films 208formed on the lower surfaces of the thin plates 206 b on the lower layerside is reduced, whereby the electrodeposition coating films 208 cformed on the rear surfaces 210 of the friction stir welding portions(the rear surfaces of the flange sections 204 a and 204 b) areprotected.

Note that the third embodiment exemplifies a case in which theelectrodeposition coating films 208 a to 208 c are formed in advance bythe electrodeposition coating processing on both the front and rearsurfaces and the joining surfaces (laminating surfaces) between both thefront and rear surfaces of the flange sections 204 a and 204 b of therespective left and right side members 22 a and 22 b. However, theelectrodeposition coating films 208 a to 208 c may be formed by theelectrodeposition coating processing after both the front and rearsurfaces and the joining surfaces (laminating surfaces) between both thefront and rear surfaces of the flange sections 204 a and 204 b are eachplated with a zinc alloy or the like.

In this case, the electrodeposition coating films 208 a, the plated zincalloys or the like, and the sealants 258 are mixed together and extrudedto the peripheries of the respective joining surfaces between the flangesections 40 and 40 of the left and right side sections 36 a and 36 b andthe flange sections 204 a and 204 b of the left and right side members22 a and 22 b, whereby the walls of the mixtures m of theelectrodeposition coating film 208 a, the plated zinc alloys or thelike, and the sealants 258 are formed. With the formation of the walls,the flange sections 40 and 40 of the left and right side sections 36 aand 36 b and the flange sections 204 a and 204 b of the left and rightside members 22 a and 22 b are firmly fixed together (solid-phasejoined) when the intermetallic compounds as the compounds of light steeland iron are formed by plastic flow. Therefore, the separation of theelectrodeposition coating films 208 a and the plated zinc alloys or thelike is prevented. In addition, the electrodeposition coating films 208a, the plated zinc alloys or the like, and the sealants 258 do not existat the respective joining surfaces between the flange sections 40 and 40and the thin plates 206 a and 206 b. Note that in the third embodiment,the sealants 258 may not be used. However, it is more desirable to usethe sealants 258 since they have antirust performance.

FIG. 26 is a plan view of a subframe structure according to a fifthembodiment. Note that in FIG. 26, reference symbols 62 s (startsections) represent locations at which the friction stir welding isstarted and reference symbols 62 e (end sections) represent locations atwhich the friction stir welding is ended. In addition, open arrowsbetween the reference symbols 62 s and 62 e represent the progress ofthe operation of the friction stir welding.

FIGS. 27A-27C are views showing the process of the friction stir weldingapplied to the subframe structure according to the fifth embodiment.FIG. 27A is a cross-sectional view showing the state of the startsection of a location at which the friction stir welding is started.FIG. 27B is a cross-sectional view showing a state before the frictionstir welding at the end section of a location at which the friction stirwelding is ended. FIG. 27C is a cross-sectional view showing a stateafter friction stir welding at the end section of a location at whichthe friction stir welding is ended.

A subframe structure 400 according to the fifth embodiment is differentfrom the subframe structure 200 according to the third embodiment inthat the shapes of the start sections 62 s at which the friction stirwelding is started and those of the end sections 62 e are changed toperform the friction stir welding shown in FIGS. 25A-25C.

As shown in FIG. 27A, concave-shaped concave sections 40 b, whichreceive the tip end of the joining pin 54 protruding downward from thebottom center of the rotor 52 and are greater or substantially equal tothe tip end of the joining pin 54, are formed in the flange sections 40and 40 of the left and right side sections 36 a and 36 b made of lightmetal such as aluminum at the start sections 62 s.

With this configuration, the generation of chips (burrs) from the flangesections 40 is reduced and the insertion of the joining pin 54 isenhanced when the joining pin 54 is rotated to penetrate to start thefriction stir welding. Accordingly, the process of the friction stirwelding can be smoothly started, and the finished quality of the startsections 62 s at which the friction stir welding is started can be madesatisfactorily.

Further, the joining pin 54 is rotated from the start sections 62 s toextrude the electrodeposition coating films 208 a to the peripheries ofthe joining surfaces between the flange sections 40 and the thin plates206 a to form the walls, is rotated in direct contact with the thinplates 206 a to continue the friction stir welding of the flangesections 40 and the thin plates 206 a and 206 b, and reaches the endsections 62 e at which the friction stir welding is ended as shown inFIG. 27C. At this time, since the flange sections 40 of the left andright side sections 36 a and 36 b and the thin plates 206 a and 206 bare firmly fixed together in such a manner that the electrodepositioncoating films 208 a are extruded to the peripheries of the joiningsurfaces between the flange sections 40 and the thin plates 206 a toform the walls, the separation of the electrodeposition coating films208 a is prevented. In addition, the electrodeposition coating films 208a do not exist at the joining surfaces between the flange sections 40and the thin plates 206 a.

Next, a description will be given of the configuration of the endsections 62 e serving as points at which the friction stir welding ofthe flange sections 40 and the thin plates 206 a and 206 b as shown inFIG. 27B and FIG. 27C is ended.

At the upper parts of the end sections 62, at which the friction stirwelding is ended, of the thin plates 206 a and 206 b ranging from thecentral sections 24 b of the left and right side members 22 a and 22 bto the extending sections 202 behind the central sections 24 b of thefront subframe 12 shown in FIG. 26, concave-shaped concave sections 202h greater than the tip end of the joining pin 54 are formed in advanceas shown in FIG. 27B. At the same time, convex-shaped convex sections 40c fitted in the concave sections 202 h of the thin plates 206 a and 206b are formed in advance in the flange sections 40 and 40 of the left andright side sections 36 a and 36 b made of light metal such as aluminum,the flange sections 40 and 40 being superposed on the thin plates 206 aand 206 b at the end sections 62 e from above.

Prior to the friction stir welding, as shown in FIG. 27B, the flangesections 40 and 40 of the left and right side sections 36 a and 36 bmade of light metal such as aluminum are superposed on the thin plates206 a and 206 b of the left and right side members 22 a and 22 b of thefront subframe 12 so that the convex sections 40 c and 40 c of theflange sections 40 and 40 are fitted in the concave sections 202 h ofthe thin plates 206 a.

Then, when the joining pin 54 protruding downward from the bottom centerof the rotor 52 is rotated to penetrate the flange sections 40 on theupper side, the flange sections 40 and 40 of the left and right sidesections 36 a and 36 b of the rear subframe 14 and the thin plates 206 aand 206 b of the left and right side members 22 a and 22 b of the frontsubframe 12 are joined together by the friction stir welding as shown inFIG. 27C.

As shown in FIG. 27B, the concave sections 202 h greater than thejoining pin 54 are formed at the upper parts of the end sections 62 e,at which the friction stir welding is ended, in the thin plates 206 aand 206 b of the left and right side members 22 a and 22 b of the frontsubframe 12. In addition, the convex sections 40 c fitted in the concavesections 202 h of the thin plates 206 a and 206 b are formed in theflange sections 40 and 40 of the left and right side sections 36 a and36 b made of light metal such as aluminum, the flange sections 40 and 40being superposed on the thin plates 206 a and 206 b at the end sections62 e from above. Thus, the exposure of the thin plates 206 at the endsections 62 e is prevented after the friction stir welding.

In addition, since the light metal such as aluminum of the flangesections 40 is filled in the concave sections 202 h of the thin plates206 a and 206 b, the occurrence of corrosion is reduced at the endsections 62 e of the thin plates 206 a and 206 b.

Note that as in the third embodiment, the electrodeposition coatingfilms may be formed on the thin plates 206 a, 206 b, and 206 c afterboth surfaces of the respective thin plates 206 a and 206 are platedwith a zinc alloy or the like.

In this case, the joining pin 54 is rotated to mix together theelectrodeposition coating films 208 a and the plated zinc alloys or thelike (antioxidants) and protrude the mixtures to the peripheries of thejoining surfaces between the joining pin 54 and the thin plates 206 a toform walls, and is rotated in contact with the thin plates 206 a to jointhe flange section 40 and the thin plates 206 a and 206 b together bythe friction stir welding. On this occasion, while the electrodepositioncoating films 208 a and the plated zinc alloys or the like are extrudedto the peripheries of the joining surfaces to form the walls,intermetallic compounds are formed to firmly fix together the flangesections 40 of the left and right side sections 36 a and 36 b and thethin plates 206 a. Therefore, the separation of the electrodepositioncoating films 208 a and the plated zinc alloys or the like is prevented.In addition, the electrodeposition coating films 208 a and the platedzinc alloys or the like do not exist at the joining surfaces between theflange sections 40 and the thin plates 206 a.

Note that as shown in FIG. 27C, it is desirable to apply the sealants 58(as indicated by two-dot chain line) onto the thin plates 206 a beforesuperposing the flange sections 40 on the thin plates 206 a. In thiscase, while the sealants 58, the electrodeposition coating films 208 a,and the plated zinc alloys or the like are extruded to the peripheriesof the joining surfaces to form the walls, the joining pin 54 is rotatedin contact with the thin plates 206 a to firmly fix together the flangesections 40 of the left and right side sections 36 a and 36 b and thethin plates 206 a. Therefore, the sealants 58, the electrodepositioncoating films 208 a, and the plated zinc alloys or the like do not existat the joining surfaces between the flange sections 40 and the thinplates 206 a.

In addition, the configurations of the start sections 62 s and the endsections 62 e in the process of the friction stir welding according tothe fifth embodiment may be applied to the first to fourth embodiments.

According to the configurations of the first to fifth embodiments, thefriction stir welding is performed in a state in which the cation (ED)electrodeposition coating or the like is applied. Therefore, desiredjoining strength can be ensured.

In joining the light metal member such as an aluminum member and theiron member together by the friction stir welding, the friction stirwelding is performed after the iron member is subjected to theelectrodeposition coating in advance. Thus, the coating films are notmelted as in melt welding, which saves the labor of applying coatingsand allows the application of coatings to be preceded in every detail.In addition, the light metal member and the iron member can be joinedtogether by the extrusion of the coating films to the outside.

Note that the above embodiments describe the cation electrodepositioncoating as electrodeposition coating, but electrodeposition coatingother than the cation electrodeposition coating may be applied.

Note that the above embodiments describe the aluminum alloy (aluminum)as an example in which the rear subframe 14 is made of light metal, butit is needless to say that light metal other than the aluminum alloy(aluminum) may be used.

In addition, the first to fifth embodiments describe the variousconfigurations, but the respective configurations may be arbitrarilycombined together as occasion demands.

Note that the embodiments exemplify the zinc alloy plating, but purezinc plating may be used. However, the zinc alloy plating is moredesirable since it is more excellent in moldability and anticorrosion.Note that “zinc plating,” which will be described later, includes boththe zinc alloy plating and the pure zinc plating.

With the joining method described above, the following advantages oreffects are obtained.

The method of superposing a steel member and a light metal member one onthe other and joining them together by the friction stir welding intheir non-melting state may include a coating process in which the steelmember is coated and a joining process in which a rotation tool isrotated to penetrate the joining section between the light metal memberand the steel member and the joining section of the light metal memberis softened and plastically flowed by friction heat generated at thistime to join the steel member and the light metal member together.

According to this joining method, the application of a coating can bepreceded in every detail since a coating film is not melted as in meltwelding. In addition, the application of a coating can be preceded sincethe coating film can be extruded outward. Moreover, an intermetalliccompound is formed by plastic flow.

In addition, the coating film may not exist on the joining surface insuch a manner that the application of a coating is performed based onelectrodeposition coating and the coating film by the application of acoating is extruded to the periphery of the joining surface between thelight metal member and the steel member. According to the joiningmethod, the light metal member and the steel member can be joinedtogether by the extrusion of the coating film to the outside of thejoining surface.

Further, the steel member may be plated with zinc, and a layer platedwith the zinc may be extruded to the periphery of the joining surfacetogether with the coating film of the electrodeposition coating.According to this joining method, the light metal member and the steelmember can be joined together by the extrusion of the layer plated withthe zinc.

Furthermore, a sealant may be provided between the steel member and thelight metal member and extruded to the periphery of the joining surfacetogether with the layer plated with the zinc and the coating film of theelectrodeposition coating. According to this joining method, the sealantis mixed with the coating film and the antioxidant of the plated zinc ofthe steel member, whereby an antirust effect can be exhibited. Inaddition, the steel member and the light metal member can be joinedtogether by the extrusion of the mixture of the sealant and othersubstances to the outside of a joining interface.

Furthermore, the tip end of the rotation tool may be pushed until it isbrought into contact with the steel member. According to this joiningmethod, the light metal member can be reliably stirred, and the layerplated with the zinc, the coating film, and the like can be extruded ifthe steel member is coated with the layer plated with the zinc, thecoating film, and the like.

Furthermore, the steel member at the joining section may include aplurality of steel members superposed one on another. According to thisjoining method, an increase in the temperature of the lower surface ofthe steel member can be reduced in the process of joining the steelmember and the light metal member together.

Furthermore, a first concave-shaped concave section greater than orsubstantially equal to the tip end of the rotation tool may be formed inthe section of the light metal member in which the tip end of therotation tool is caused to penetrate at a start section at which thejoining process is started. According to this joining method, theinsertion of the tip end of the rotation tool into the steel member isenhanced, whereby the generation of chips can be reduced.

Furthermore, a second concave-shaped concave section greater than thetip end of the rotation tool may be formed in a section of the steelmember in which the tip end of the rotation tool is caused to penetrateat an end section at which the joining process is ended, and aconvex-shaped convex section received in the second concave section ofthe steel member may be formed in the light metal member. According tothis joining method, the exposure of the steel member can be preventedat the end section at which the joining process is ended, and theoccurrence of corrosion can be reduced since the steel member at the endsection can be coated with the light metal member.

EXPLANATION OF REFERENCES

-   -   10, 100, 200, 300, 400: subframe structure    -   11: vehicle    -   12: front subframe (member made of steel, steel member)    -   14: rear subframe (aluminum member, light metal member)    -   20: front cross member    -   22 a, 22 b: left side member, right side member    -   26: thin plate section (extending section)    -   28: flange section (steel member)    -   30: bolt    -   32: bolt insertion hole    -   36 a, 36 b: left rear side section, right rear side section    -   38: rear cross section    -   40: flange section (light metal member)    -   40 b: concave section (first concave section)    -   40 c: convex section    -   44: closed cross section    -   54: joining pin (rotation tool for friction stir welding,        rotation tool)    -   58, 258: sealant (sealing member)    -   62 s: start section    -   62 e: end section    -   102 a, 102 b: thin plate    -   202 h: concave section (second concave section)    -   204 a, 204 b: flange section    -   206 a, 206 b: steel thin plate (a plurality of superposed steel        members)    -   208 a to 208 c: electrodeposition coating film    -   210: rear surface    -   S1: setting of workpiece (coating process)    -   S4: friction stir welding and extrusion of the sealant (joining        process)

1. A subframe structure for a vehicle, the subframe structure beingdisposed at a front of the vehicle and fixed to or floatably supportedby a vehicle-body member, the subframe structure comprising: a lightalloy member comprising a left side section and a right side sectioneach extending in a front-rear direction of the vehicle, and a crosssection extending in a width direction of the vehicle; and a steel platemember comprising a left thinned plate section and a right thinned platesection each extending in the front-rear direction of the vehicle,wherein: the left side section and the right side section include at atleast peripheries thereof a first left flanged section and a first rightflanged section, respectively; the left thinned plate section and theright thinned plate section include at at least peripheries thereof asecond left flanged section and a second right flanged section,respectively; with the light alloy member stacked on the steel platemember, the first left flanged section and first right flanged sectionare joined to the second left flanged section and second right flangedsection by friction stir welding from a light alloy member side,respectively, thereby forming a closed section; the left side sectionand the cross section are connected together by a left connectingportion; the right side section and the cross section are connectedtogether by a right connecting portion; the first left flanged sectionand the second left flanged section are joined together by opposed frontleft joint portions and opposed rear left joint portions using frictionstir welding at front and rear sides of the left connecting portion,respectively; and the first right flanged section and the second rightflanged section are joined together by opposed front right jointportions and opposed rear right joint portions using friction stirwelding at front and rear sides of the right connecting portion,respectively.
 2. The subframe structure according to claim 1, whereinthe left connecting portion, the right connecting portion and the crosssection project upwardly, so that the light alloy member as a wholeserves as an opening section structure.